Mechanisms of Memory Triage during Sleep

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At first glance, sleep appears to be a maladaptive step of evolution. Sleep imposes a prolonged period of "inactivity", reducing the time an animal can spend on foraging, hunting, and finding/attracting mates. Furthermore, sleep is substantially more dangerous than staying awake, as the very sensory inputs protecting us from predation are suppressed. So, what then do we get in return? The short answer is that we do not know. It is clear that evolution has made sleep a biological requirement. Your brain will not let you voluntarily avoid sleeping more than a few nights, and involuntary sleep deprivation leads to chronic stress, fatigue, disorientation, poor health and even death. However, in spite of decades of research, we still do not fully understand why we have this biological need to sleep.Sleep likely serves a multitude of roles benefiting from a prolonged "offline" state with reduced sensory inputs. One major hypothesized function is systems-level memory consolidation, a process by which new memories stored in the hippocampus are stabilized and transferred to neocortical brain regions for long-term storage. One key aspect of memory consolidation is the phenomenon of memory triage, by which salient or repeated experiences are more likely to be consolidated compared to irrelevant experiences. How the brain accomplishes this selective strengthening of important memories is unknown, and the focus of our research proposal.Sequential patterns of neural activity occurring during an experience have been observed to spontaneously reactivate during sleep. This reactivated memory trace, referred to as replay, is postulated to be how the brain consolidates a memory, with the transfer of a memory residing in the hippocampus to other brain regions. We will examine how replay is modulated by an experience's salience, and hypothesize that more important experiences will replay more during sleep. Furthermore, we predict that cortical feedback plays a key role in modulating replay based on salience. To test this, we will inactivate cortical inputs to the hippocampus and examine whether the hippocampus no longer biases replay towards more salient experiences.Approximately 70% of Brits sleep less than the 7-9 hours per night recommend by the medical community. Furthermore, increased social media use is correlated with poorer sleep quality, along with an increased occurrence of anxiety and depression. As the amount of "junk information" in our lives increases, and the amount we sleep decreases, there is a growing pressure on our memory triage system to effectively prioritize the consolidation of salient information, and an increased need to understand how this occurs. These issues are exasperated further in older individuals, as both sleep quality and duration decline with age. Our research will directly examine the underlying mechanisms of memory triage, and provide new ideas on how this process could be further optimised.